The present invention relates to a method for determining the energy efficiency of the energy system in a hybrid vehicle.
Vehicles and vehicle components are tested during manufacture of the vehicle. Most components can be tested and verified at the sub-supplier when the component is manufactured. However, many components are comprised in a system in which several different components make up the system. Such a system can only be tested and verified when completely assembled. Some system can only be tested and verified when mounted on the vehicle, since they require input from other systems.
Some components and systems have greater deviations than other. It is e.g. fairly easy and cost-effective to manufacture valves within tight tolerances, but more difficult or impossible to manufacture an electrical component within a specified tolerance, especially batteries. The only way to arrive at a required tolerance may then be to manufacture several components and to thereafter measure all components and to group them in different component classes having different tolerances. This is a costly and time-consuming operation. Further, this grouping may only be valid when the component is new, since different components may age differently. Even if a new component was within tolerance, it may be seen as out of tolerance when measured after use. In order to comply with given tolerances or requirements, some components or systems are mounted on the vehicle and are thereafter adjusted to a specific tolerance.
Some components are known to degrade with age. The exact degradation may not be known, but for some components or systems, it is possible to predict a fairly reliable degradation, e.g. by taking account on environmental factors, time and amount of use. For some components, this works fine, e.g. for mechanical components. For some components, e.g. lamps, it is more or less impossible to predict the degradation and the end of life when the lamp breaks.
For other components, like batteries, it is possible to predict the degradation with a fairly high precision, when external parameters are used to modify the degradation behaviour. Such parameters that affect the degradation are e.g. temperature, state of charge, total current flow in and out of the battery, maximal current flow in and out of the battery, life length, etc. The condition of a battery is often referred to as the State of health (SOH), where the actual condition of a battery, e.g. a battery cell or a battery pack, is compared to its ideal condition. The unit of SOH is percent points, where 100% means that the actual battery condition match the battery specification.
Normally, the SOH will be 100% at the time of manufacture and will decrease over time and use. However, the performance of a battery at the time of manufacture may not meet its specifications, in which case the initial SOH will be less than 100%. Some batteries may also have a SOH that exceeds 100%. In order to monitor the condition of the batierv in a vehicle, the SOH is monitored or estimated during use. Different parameters are used to estimate and define the SOH. The following parameters can be used singly or in combination to define a value for the SOH of a battery: internal resistance, capacity, voltage, self-discharge, ability to accept a charge or the number of charge-discharge cycles performed. Different manufactures may use a different definition of the SOH. When several parameters are used, weight for each of the parameter's contribution to the SOH value may also differ.
In order to protect the battery from overload, an estimated degradation of the SOH is applied to the battery, which means that capacity of the battery is decreased over time. If the SOH for a new battery is 100%, it may be set to 80% after one year. The advantage of limiting the SOH of a used battery is to protect it. By limiting the maximum power into the battery, an overload of the battery can be avoided. However, since the degradation of the SOH is estimated and since this degradation is equal for all similar batteries, the estimation, must be on the safe side such that no batteries are overloaded. This may lead to that some batteries are limited more than necessary.
It would thus be of advantage to find a way to determine the efficiency of an energy system of a hybrid vehicle over time.
It is desirable to provide an improved method for determining the efficiency of an energy system of a hybrid vehicle over time, it is also desirable to provide a hybrid, vehicle comprising a control unit adapted to determine the efficiency of an electric energy system of the vehicle.
In a method for determining the efficiency of an electric energy system of a hybrid vehicle comprising a combustion engine, an electric machine, service brakes and an electric storage system, the steps of determining a requested brake energy for a first vehicle retardation and determining an actual electrical brake energy in an electric machine of the hybrid vehicle and an actual mechanical brake energy in service brakes of the hybrid vehicle during the vehicle retardation, determining a measure representative of a relation between, the electrical brake energy and the mechanical brake energy, determining a new measure representative of a relation between the electrical brake energy and the mechanical brake energy at a subsequent similar vehicle retardation, comparing the determined measures representative of a relation between the electrical brake energy and the mechanical brake energy of the first and second vehicle retardations, where the actual efficiency of the energy system is obtained from, the change in the measure representative of a relation between the electrical brake energy and the mechanical brake energy of the first and second vehicle retardations are comprised. By this first embodiment of the method, the method can determine the efficiency of an energy system of a hybrid vehicle. The energy system is preferably a battery connected to an electric machine. The electric machine is used as an electric motor to power the vehicle and as a generator to charge the battery when the vehicle retards, it is also possible to charge the battery when the vehicle is powered by the combustion engine and is travelling on a relatively even road. When the vehicle retards, the electric machine will be used as an electric brake that regenerates the battery. When the SOH of the battery is high, i.e. when the battery is new, the battery will be able to receive more power than when the battery has been used for a few years. By e.g. comparing the ratio of available electrical brake energy and mechanical brake energy, the actual efficiency or SOH of the battery can be determined instead of using a predefined degradation figure for the SOH. In this way, the battery of a vehicle can be monitored individually, and the charging/discharging properties can be adapted to the actual condition of the battery. This may allow for a prolonged life of the battery. In further embodiments, the measure representative of a relation between the electrical brake energy and the mechanical brake energy can be one of:                the ratio between the electrical brake energy and the mechanical brake energy;        the ratio between the requested brake energy and the mechanical brake energy, and;        the ratio between the requested brake energy and the electrical brake energy.        
The measure that is representative of a relation between the electrical brake energy and the mechanical brake energy can be expressed and calculated in different ways. One measure may e.g. be the ratio between the electrical brake energy and the mechanical brake energy for a retardation. Another measure may be the difference between the electrical brake energy and the mechanical brake energy for a retardation. Further suitable measures are e.g. the ratio between the electrical brake energy and the total brake energy or the ratio between the mechanical brake energy and the total brake energy. It would also be possible to relate the electrical brake energy or the mechanical brake energy to the requested brake energy.
The measure representative of a relation between the electrical brake energy and the mechanical brake energy may be determined by e.g. determining the requested brake power for a retardation and by determining the actual electrical brake energy and the actual mechanical brake energy during the retardation. The requested brake power is available from the control system of the vehicle and may be a measure of how hard the brake pedal is pressed by the driver. The requested brake energy may be obtained from the requested brake power integrated over the total brake time. If the vehicle is provided with other auxiliary brakes, the total sum of all requested brake power is used. The actual electrical brake energy is determined by measuring the current flow into the battery from the generator and by measuring the voltage over the battery. The actual mechanical brake energy can be obtained from the difference between the requested brake power and the actual electrical brake energy. The actual mechanical brake energy can also be obtained in other ways. e.g. by measuring the pressure in the brake system, by a displacement sensor in the brake or by measuring the wear of the brake pads.
The retardation of the vehicle is preferably relatively strong, such that the electric brake is used at its maximum and such that a part of the brake power comes from the mechanical brake. Normally, the electric machine is dimensioned such that it is intended to work at its upper limit most of the lime during normal use of the vehicle. When the electric machine is used as an electric motor, it will reach its maximum torque relatively fast and will then continue to deliver this torque until a change in the driving pattern, e.g. a gear shift, is performed. When the electric machine is used as a generator during a vehicle retardation, the generator will relatively fast deliver its maximum current to the battery and will continue to do so until the vehicle has stopped, if more brake power than the electric brake can deliver is required, an auxiliary brake will be used. Normally, the service brakes of the vehicle will be used as the auxiliary brake, but it would also be possible to use other types of brakes, such as an exhaust brake if the engine is running or a retarder of some kind.
For the determination of the efficiency of the energy system, the subsequent retardations should be similar, such that the results can be compared. One way to determine when a subsequent vehicle retardation is similar to the first vehicle retardation is to use the requested brake power combined with some vehicle parameters. The weight of the vehicle may e.g. be used as a vehicle parameter. It is not required that the weight of the vehicle is similar, but with an empty vehicle, less brake power is required to stop the vehicle. It is thus of advantage if the weight of the vehicle is in the same range for the different retardations. The speed of the vehicle at the start of the retardation may also be used as a vehicle parameter. Two retardations may be seen as similar if the required brake power differs less than 20%.
Another vehicle parameter may be the position of the vehicle. Most heavy hybrid vehicles are used as distribution trucks or passenger busses. Such vehicles often travel the same routes. They will thus retard frequently at the same position. A bus will e.g. stop at the same bus stop or will travel down the same downhill slope frequently. The position of the bus stop or the slope can thus be used to determine when a similar retardation is performed, if the time of day is also used as a parameter, the load of the bus will most likely be similar at the same time of day. In rush hours, the bus may be fall and during noon or late evenings, the bus may be almost empty. Delivery trucks often travel the same route and deliver the same amount of goods at specific locations.
By determining a measure, such as a ratio or difference, representative of a relation between the electrical brake energy and the mechanical brake energy of a first and a second vehicle retardation, a measure of the actual efficiency of the energy system can be obtained. It is of advantage if the time period between the retardations is relatively long, such that a measurable difference can be detected. The time period between the first and the second retardation is preferably several months or more. The measured retardation values are preferably stored in a memory.
It is also possible to save several subsequent retardations and to comparing these over time, such that a time dependent curve for the actual efficiency of the energy system is obtained. When the energy system is a battery, a measure of the SOH of the battery can be derived from the efficiency. One advantage of determining a true measure of the SOH is that the battery can be used in an optimal way taking account of the actual condition and not on a predicted condition. This is of advantage since different batteries age in different ways, depending on the actual use and on the build quality.
When the battery has degraded with use after one or a few years, such that, they cannot receive as much current as a new battery, the mechanical brakes will be used more during a retardation which means that the wear of the mechanical brakes will increase. This wear can also be used as a measure of how much the battery has degraded. The wear of the brake pads over time can be measured, e.g. by a displacement sensor.